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  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
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  <script>
    var countCollisions = function (directions) {
      //去掉开始的'L'
      while (directions[0] == 'L') {
        directions = directions.slice(1)
      }
      //去掉结尾的'R'
      while (directions[directions.length - 1] == 'R') {
        directions = directions.slice(0, directions.length - 1)
      }
      //去掉'S'
      directions = directions.replaceAll('S', '')
      return directions.length
    };
    console.log(countCollisions(directions = "RLRSLL"))
  </script>
</body>

</html>